Filter system



April l, 1947.

` E. LABIN i FILTER SYSTEM Filed Dec. 1', 142

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INVENTOIL EMILE LHB/N April 1, 1947. E, L^B|N 2,418,127

FILTER SYSTEM Filed Dec. l, 1942 3 Sheets-Sheet 2 [4185 I-v LI@ I Y MA l Al Patented Apr. 1, 1947 UNITED STATES FILTER SYSTEM Emile Labin, New York, N. Y., assigner to Federal Telephone and Radio Corporation, Newark, N. J., a corporation of Delaware Application December 1, 1942, Serial No. 467,509

14 Claims. 1

This invention relates to radio impulse systems and more particularly to systems for receiving and filtering or discriminating between impulses of different width characteristics.

In the case of impulse transmission where the impulses are modulated for transmission of intelligence or transmitted for obstacle detection purposes, additional impulses may be imposed upon the carrier wave by an enemy to interfere and jam the impulse transmission. Additional impulses differing in character such as width may be added to the carrier Wave to confuse the enemy and render it diflicult for him to determine which of the impulses of the wave carries intelligence or is used for obstacle detection purposes. Then again the transmitting system may be arranged to transmit a series of differently shaped impulses each representing a different channel of intelligence. In other words, each series of impulses may distinguish from the other series in width or some other characteristic, and be modulated with a different intelligence thereby providing plural channel transmission.

Where impulses carrying intelligence are transmitted at a given frequency or in a given time relationship, interfering or-additional impulses, except in the case of superposition, can be eliminated by blocking. A system providing such blocking to eliminate unwanted impulses is disclosed in my copending application Serial No. 386,282, filed April 1, 1941. Should the impulses carrying intelligence be transmitted at irregular timing, this frequency or time selection method of blocking is not entirely satisfactory.

It is an object of this invention, therefore, to provide a method and means to selectively lter a wave modulated with impulses differing in a characteristic such as Width so as to eliminate all impulses differing from a given width.

The method of this invention comprises, for example, iirst time differentiating the pulse modulated wave to obtain a first series of pulses made up of pulses of a given polarity such as positive corresponding to the leading edges of the impulses and pulses of opposite polarity such as negative corresponding to the trailing edges thereof. I then produce a second series of pulses, including at least pulses of negative polarity, one each disposed from one of the positive pulses of the rst series a time interval equal to a given Width impulse the detection of which is desired. This second series of pulses is combined with the positive and negative pulses of the first series. Since the negative pulses of the second series coincide with the negative pulses produced from different methods.

the wanted impulses, they add resulting in a pulse having an amplitude equal to the sum of the coinciding pulses. The resulting pulse energy is clipped at a threshold level selected beyond the amplitude of the pulses before the combining operation so as to obtain a pulse output in accordance with the occurrence of the impulses wanted for detection.

The second series of pulses may be produced by One of the methods may comprise producing a series of impulses of the given width in synchronism with the positive pulses and then differentiating the series of impulses of given width thereby producing a second series of alternate positive and negative pulses,

the latter of which are the negative pulses to be combined with the negativepulses of the rst series.

Another method of producing the pulses of negative polarity comprises inverting the first series of positive and negative pulses and then retarding for a time interval equal to the width of the selected impulses the pulses thus inverted. The retarded pulses are combined with the positive and negativeA pulse energy of the first series so that inverted positive pulses add to the negative pulses of those impulses having the width of the selected impulse. A subsequent clipping operation at an amplitude level beyond the amplitude of the pulses before the combining operation produces a pulse output corresponding to the occurrence of the pulses of the desired Width. The inversion of the original negativepulses will provide additional positive pulses in the combining operation, but since the clipping occurs on the negative side of the wave, the

addition of pulses on the positive side doesnt matter.

For a better understanding of the method,

40 reference may be had to the following detailed description of systems by which the method may be practiced, to be read in connection 'with the accompanying drawings, in which;

Fig. 1 is a block diagram of an impulse filtering system in accordance with this invention;

Fig. 2 is a schematic wiring diagram of the system shown in Fig. 1;

Fig. 3 is a graphical illustration of the steps performed by the system shown in Figs. 1 and 2;

Fig. 4 is a block diagram of another system by Which the method of this invention may be practiced, and;

Fig. 5 is a graphical illustration of the steps performed by the system shown in Fig. 4.

Referring to Figs. 1, 2 and 3, an impulse moduimpulses A, B, C, and D. The desired impulse or' channel to be detected may be represented by the impulse width A. It may also be assumed that lthe impulses A occur irregularly so that frequency and time selection by blocking is not a satisfactory method of eliminating the unwanted impulses B, C and D.

The impulses of the wave I2 are first diiferentiated with respect to time by a differentiating circuit I4 comprising a condenser I4a and a resistor I4b. This results in the production of a1` ternate positive and negative pulses Ai and A2, B1 and B2, etc., for each of the impulses of the wave I2. Energy of the positive and negative pulses thus produced is applied to a multi-vibrator I6 in which the cathodes of the tube circuits thereof are provided with a variable resistor I1 whereby the multi-vibrator is adjustable to produce im pulses I8 of a given width in response to each of the positive pulses A1, Bi, etc. By adjusting the resistor Il to a desired value, the multi-vibrator is adapted to be triggered from one state of operation bythe positive pulses to another state ofA operation, and due to the biasing of the resistor 'I'I to reurn to the first state of operation a given time interval after the triggering thereof. This produces impulses I8 of a desired width and for the present illustration a width is selected equal to the width of the wanted impulses A.

The multi-vibrator outlet I9 is connected to a differentiating circuit comprising a, condenser 20a and a resistor 20h. This produces alternate positive and negative pulses I8a and I8b as indicated in Fig. 3. The positive and negative pulses A1 and A2, etc., and the-pulses |811 and I 8b, etc., are combined in a circuit 22 having a vacuum tube 23 which preferably is a pentode. The pentode circuit 22 is arranged so that the energy of the two series of pulses is mixed and the pulses coinciding in time are combined, and the resulting enery thereof is permittedto pass through the anode connection 25 when the combining Apulse energy exceeds a predetermined amplitude determined by the biasing of the tube circuit. The combining action of the circuit is illustrated by the curve 30. It will be readily observed that where the impulses A occur the negative pulses Az and I8 combine to produce a pulse 3| which is of greater amplitude than the uncombined pulses. The positive pulses A1 and it, B1 and I8'. etc., of course, combine to produce pulses 32 of amplitude similar to the amplitude of the negative pulses 3|. Since it is desirable to obtain pulses corresponding to the pulses A only, a threshold clipping operation as indicated by the line 34 produces a pulse output 35 which corresponds with the occurrence of the impulses A. It will thus be apparent that by my method the unwanted impulses B, C and D are eliminated and pulses corresponding to the wanted impulses A are obtained. Should the impulses A be modulated in time displacement the pulse output 35 will contain the displacement and the intelligence ,represented by the modulation can be obtained by known means.

Any desired impulse width may be selectively obtained .by the system by proper adjustmentof the biasing resistor I'I.

4 width is obtained by adding .to pulses corresponding to the trailing edge of the desired impulse other energy of 'the same polarity. In the example given, the trailing edge is indicated by a negative pulse and therefore the added pulse is also negative. trailing edge is positive in polarity the added pulse must be of the same polarity.

Referring to Figs. 4 and 5, another system is shown by which the method of this invention may be practiced. To illustrate that the impulses of the modulated wave source may vary in shape other than width and yet be filtered for a given width pulse only, I have shown a wave 40 with impulses E, F, G and H which vary in shape, amplitude and width. Most transmitting means, however, provide substantially constant amplitude impulses and impulses which vary in the manner shown in Fig. 5 are not usually transmitted by one system. This wave form, however, is shown for purposes of illustrating' that pulses of a given Width may be filtered in accordance with this invention either by the system shown in Figs. 1 to 3 or Figs. 4 and 5 regardless of the other characteristics thereof. Assuming that the impulses E are the wanted pulses, the system will be adjusted to select pulses of a time duration corresponding to the width of the impulses E. 'Ihis is accomplished by providing the system with a vmade negative and the negative pulses are made positive. These inverted pulses are then retarded in accordance with the adjustment of the delay device 45 so that the inverted pulses assume a time relation indicated by the pulses Eix, Ezx and Fix, etc., in curve 45. The energy of the inverted and retarded pulses is then mixed and combined as the case may be with energy of the positive and negative pulses produced at the output of the differentiating device 43 in a combining device 41 similar to the circuit 22 of Fig. 2. The device 41 is biased so that the resulting energy is threshold clipped on the negative side of the wave as indicated by the line 48. This produces output pulses Eo corresponding to the wanted impulses E.

While I have described above the principles of the method of my invention and two specific systems for' practicing the method, it will be clearly understood that this description is made only by' Way of vexample and not as a limitation on the scopeof my invention as set forth in the objects thereof and the appended claims..

What I claim is:

1. A method of selectively ltering a wave formed of different width impulses to produce pulses corresponding in time to the impulses of av given width only, comprising differentiating the Wave to obtain a series of derivative pulses of a given polarity'corresponding to the leading edges of the impulses and a second series of derivative pulses of opposite polarity corresponding to the trailing edges thereof, producing selecting pulses having a polarity corresponding to the polarity of one of said series of derivative pulses, said selecting pulses being disposed a time interval equal to said given width from derivative If, however, the pulse defining the Energy of these alterpulses of one of said series, mixing said derivative and selecting pulses, and clipping the resulting pulse energy at an amplitude level beyond the amplitude of the selecting pulses.

2. The method dened in claim 1 wherein the production of said selecting pulses comprises producing impulses of said given width in synchronism with the derivative pulses of said given polarity only.

3. The method defined in claim 1 wherein the production of said selecting pulses comprises inverting and retarding energy of the derivative pulses of said given polarity, and the retardation thereof being selected equal to said given width.

4. The method dened in claim 1 wherein the derivative pulses of given polarity are positive and the pulses of opposite polarity are negative.

5. A system for selectively ltering a wave formed of diilerent widthimpulses to produce pulses corresponding in time to the impulses of a given width comprising means to differentiate the wave to produce derivative pulses of a given polarity corresponding to the leading edges of the impulses and pulses of opposite polarity corresponding to the trailing edges thereof, means to produce selecting pulses of said opposite polarity disposed a time interval equal to said given width from a pulse of said given polarity, means forl mixing the derivative and selecting pulses, and means for clipping the resulting pulse energy at an amplitude level beyond the amplitude of the selecting pulses.

`6. The system deiined in claim 5 wherein the means for producing the selecting pulses comprises means responsive to the pulses of said given polarity to produce impulses of said given Width and means to differentiate said impulses of given width thereby producing alternate pulses having said given and said opposite polarity.

7. The system defined in claim 5 wherein the means for producing the selecting pulses comprises a multi-vibrator, means to cause the multivibrator to change from one state of operation to another in response to the pulses of said given polarity and to return to said one state of operation after a predetermined time interval to produce impulses of said given width, and means to differentiate said impulses of' said given width, thereby producing alternate pulses having said given and said opposite polarity.

8. The system dened in claim 5 wherein the means for mixing the pulses comprises a vacuum tube and whereinthe means for clipping the resulting pulse energy includes means for biasing the tube to cut-oil' at a selected clipping level.

9. The system defined in claim 5 wherein the means for producing the selecting pulses in the time relation specified comprises means to invert said pulses of given polarity and means to retard the inverted pulses a selected time interval equal to said given width.

10. A method ci' producing a pulse output timed according to wave portions oi a siven duration determined by relatively abrupt changes in the amplitude of a wave, comprising differentiating the wave to obtain derivative pulses corresponding to abrupt changes in amplitude of said wave, the derivative pulses corresponding to changes increasing in amplitude being of a rst polarity and those corresponding to changes decreasing in amplitude being of a second polarity,` producing selecting pulses disposed at a time interval equal to said given duration from the derivative pulses of one of said polarities, and producing a pulse output each time a selecting pulse coincides with a derivative pulse of the other of said polarities.

11. The system dei-ined in claim 10 wherein the operation of producing selecting pulses includes inverting the energy of the derivative pulses and retarding the inverted pulses an amount equal to said given duration.

12. A system for producing a pulse output timed according to wave portions of a given duration determined by relatively abrupt changes in the amplitude of a wave, comprising means for differentiating the wave to obtain derivative pulses corresponding to abrupt changes in amplitude of said wave, the derivative pulses corresponding to changes increasing in amplitude being of a first polarity and those corresponding to changes decreasing in amplitude being of a second polarity, means to produce selecting pulses each disposed at a time interval equal to said given duration from the derivative pulses of one of said polarities, and means responsive to coincidence of a selecting pulse and derivative pulse of the other of said polarities to produce a pulse output.

13. The system defined in claim 12 wherein the means for producing the selecting pulses include means tolinvert energy of said derivative pulses and means to retard the inverted pulses a time interval equal to said given duration.

14. The system dened in claim 12 wherein theA means for producing the selecting pulses include a multi-vibrator, means to cause the multi-vl- .brator to change from one state of operation to EMILE LABIN.

REFERENCES CITED The following references are oi' record ln the le of this patent:

UNITED STATES PATENTS Name Date Reeves Dec. 16, 1941 Wheeler et al. Aug. 20, 1940 Holmes Aug. 5, 1941 the said selecting pulses.

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